When performing an induction hardening treatment on a plurality of inclined surfaces of a workpiece by induction heating or the like, it is sometimes difficult to simultaneously perform the hardening treatment on all of the inclined surface. For example, when performing the hardening treatment on two inclined surfaces 12a, 12b of a workpiece configured in a tapered manner as shown in FIG. 4, the inclined surface 12a on one side of the workpiece is firstly heated by an induction heating coil 10 and the heated inclined surface 12a is then quenched. Next, the workpiece is turned upside down, and the inclined surface 12b tin the opposite side is induction-heated in a similar manner (see Patent Document 1).